The Raman Spectrum of Janus Transition Metal Dichalcogenide Monolayers WSSe and MoSSe

Janus transition metal dichalcogenides (TMDs) lose the horizontal mirror symmetry of ordinary TMDs, leading to the emergence of additional features, such as native piezoelectricity, Rashba effect, and enhanced catalytic activity. While Raman spectroscopy is an essential non-destructive, phase- and composition-sensitive tool to monitor the synthesis of materials, a comprehensive study of the Raman spectrum of Janus monolayers is still missing. Here, we discuss the Raman spectra of WSSe and MoSSe measured at room and cryogenic temperatures, near- and off-resonance. By combining Raman data with calculations of the phonon dispersion and using symmetry considerations, we identify the four first-order Raman modes and higher-order two-phonon modes. Moreover, we observe defect-activated phonon processes, which provide a route toward a quantitative assessment of the defect concentration and, thus, the crystal quality of the materials synthesized. Our work establishes a solid background for future research on material synthesis, study and application of Janus TMD monolayers.

In contrast to conventional, mirror-symmetric TMDs with stoichiometric formula M X 2 (where M is a transition metal and X is a chalcogen), Janus TMD monolay-ers M XY are formed when the crystal plane of transition metal atoms is sandwiched between two planes each made of a different chalcogen atom X and Y . This breaks mirror symmetry along the direction perpendicular to the plane of the 2D material, reducing the overall symmetry of the crystal, and gives rise to an intrinsic electrical dipole in the unit cell created by the difference in electronegativity between top and bottom chalcogen atoms [48]. Consequently, theoretical studies predict the appearance of a multitude of physical phenomena such as piezoelectricity [49,50], enhanced photocatalysis [51][52][53][54], Rashba splitting [55][56][57], and the presence of topological phases [58].
With first reports on the successful synthesis of Janus monolayers of MoSSe having appeared in 2017 [59,60] and, to our knowledge, only one report of the growth of Janus monolayer WSSe as well as Janus heterostructure assembly [61], most of these effects are experimentally still largely unexplored. Inelastic light scattering is a powerful, non-destructive tool to gain insight into the structural and electronic properties of materials [62,63]. Each Raman spectrum of 2D materials is a unique fingerprint of a sample, shedding light on its crystal and electronic band structure [64], layer number [65], interlayer coupling [66,67], doping [68], defect density [69], electron-phonon interaction [70], etc. Moreover, Raman spectroscopy can be used in-situ during growth to distinguish a Janus monolayer from a disordered ternary alloy [71][72][73][74]. Thus, to synthesize high-quality Janus TMD crystals and unlock the predicted effects and applications it is crucial to have a detailed study of its vibrational spectrum. However, a comprehensive study of the Raman spectrum of Janus monolayers WSSe and MoSSe is lacking. Initial experimental measurements have given limited insight on the Raman spectrum of Janus TMD monolayers, suffering from incomplete [59] or even incorrect [60] assignments of the first-order modes and leaving all other features unidentified.
In this work, we calculate the phonon band structure of Janus WSSe and MoSSe monolayers and their phonon density of states (PhDOS), which we use to predict the Raman modes and their energies. Then, we measure the Raman spectra of both materials at room and cryogenic (10 K) temperature and two excitation wavelengths λ ex , closer to and farther from excitonic resonances. By comparing theory and experiments, we identify first-order Raman modes. As the experimental spectra show rich features arising beyond the calculated first-order processes, we then discuss the mechanisms of higher-order and defect-mediated Raman modes and assign them to the relevant experimental peaks.

RESULTS AND DISCUSSION
Standard TMD monolayers have D 1 3h space group symmetry [75,76], contrasting strongly with Janus TMD monolayers, for which rotation C 2 , improper rotation S 3 , and mirror σ h symmetries are broken due to the different chalcogen atoms in the unit cell. This results in a lowering of the symmetry of the crystal to the symmorphic (i.e. all symmetry operations leave one common point fixed) C 1 3v space group (C 3v point group). The unit cell of Janus monolayer M XY is formed from three atoms, resulting in 3×3 = 9 normal vibrational modes at the Γ point (centre) of the Brillouin Zone, of which three are acoustic and six are optical. Group theory identifies these vibrations as the irreducible representations of the C 3v point group, that can be expressed by Γ vib C3v = 3A 1 (Γ 1 )+3E(Γ 3 ), where all of the modes are both Raman and infrared (IR) active.

Here, Γ vib
C3v is the irreducible representation of the total vibration, deduced from the underlying crystal symmetry using the C 3v character table (see Supporting Information (SI) Table S1). In-plane vibrations are defined as E 1,2 and out-of-plane as A 1,2 1 , with E 1,2 being doubly degenerate at the Γ point (modes with the same symmetry are distinguished by the upper right corner index).
Owing to the conservation of energy and quasimomentum q in the crystal, first-order (i.e. one-phonon) scattering processes are bound to the Γ point of the Brillouin Zone due to the negligible photon momentum (q ≈ 0). The atomic displacements corresponding to the normal vibrational modes at Γ are schematically represented in Figure 1a with the transition metal atom in grey and the chalcogens Se and S in orange and yellow, respectively. We use density functional perturbation theory (DFPT) to predict the phonon modes (see Methods). In WSSe at the Γ point they occur at 204 cm −1 , 282 cm −1 , 331 cm −1 , and 420 cm −1 (for E 1 , A 1 1 , E 2 , and A 2 1 , respectively). Analogously, in MoSSe at the Γ point they occur at 208 cm −1 , 293 cm −1 , 358 cm −1 , and 445 cm −1 (for E 1 , A 1 1 , E 2 , and A 2 1 , respectively). This can be seen in the phonon band structure of monolayer WSSe in Figure 1b and of monolayer MoSSe in Figure 1c. The three acoustic phonon branches correspond to the out-of-plane acoustic (ZA), the transverse acoustic (TA) and the in-plane longitudinal acoustic (LA) modes, respectively. The remaining six branches represent the out-of-plane optical (ZO 1 and ZO 2 ), the in-plane transverse optical (TO 1 and TO 2 ) and the in-plane longitudinal optical (LO 1 and LO 2 ) modes.
In addition to the Γ point, we further examine the vibrational modes at high symmetry points at the Brillouin Zone edge, K and M. At the K point, the crystal exhibits C 3 point group symmetry with irreducible rep- , where A 1,2,3 and A 1,2,3 are both Raman active modes. The character tables for different modes at Γ, K, and M are listed in SI Section S1. Accompanied to the phonon dispersion in Figure 1b and 1c, the phonon density of states (PhDOS) reveals a high density of phonons at the flat bands, in particular close to the high-symmetry points K and M, with all phonons being Raman active. The dispersion branches of WSSe in Figure 1b are energetically lower than the dispersion branches of MoSSe in Figure 1c, thus giving lower energy of phonons at the same point in the Brillouin Zone. This mainly occurs due to the larger atomic mass of W, which makes the vibrations softer, as in the case of regular Moand W-based TMDs [77]. The two materials also differ in the values of the phonon bandgaps (see SI Section S2).
We measured the Raman spectra of Janus TMD monolayers recorded from crystals grown via room temperature Selective Epitaxy Atomic Replacement (SEAR) [61], as described in the Methods section. Here, the toplayer selenium atoms, in already grown WSe 2 and MoSe 2 monolayers, are replaced by sulfur atoms, to eventually yield Janus TMD WSSe and MoSSe, respectively. WSSe was grown on Al 2 O 3 , whereas MoSSe was grown on a Si/SiO 2 substrate. We conduct Raman spectroscopy in a back-scattering configuration, with linearly polarized excitation and no polarization filtering of the Raman signal. Figure 2 shows the Raman spectra of Janus monolayer WSSe and MoSSe between 100 and 500 cm −1 , collected with a laser excitation wavelength λ ex of 532 nm (green curves) and 633 nm (red curves), above the excitonic bandgap at 10 K of both WSSe [61] (∼670 nm) and MoSSe [59][60][61] (∼710 nm) (see SI Figure S2). Figure  2a and 2b, show typical Raman spectra recorded from Janus monolayer WSSe at room and cryogenic temperatures, respectively. From the comparison of Raman spectra at 10 K with the calculated PhDOS in Figure 2b and 2c, where the dashed blue lines indicate the calculated values of Γ phonons, we assign the first-order Raman modes E 1 at ∼208 cm −1 for λ ex = 633 nm (∼211 cm −1 for λ ex = 532 nm), . All predicted first-order Raman modes, indicated in the spectra by blue arrows, are visible in all experimental conditions, albeit their intensity is maximum at 10 K and for λ ex = 633 nm, which is close to A exciton resonance (top valence band to conduction band, see SI Figure S2). The experimental results closely match the theoretical predictions. For the spectra ac-quired at λ ex = 633 nm, we observe a broad background signal above ∼350 cm −1 , stemming from the photoluminescence tail of the material, due to the energetic proximity to the exciton transition. The Raman peaks E 1 at ∼204 cm −1 and A 2 1 at ∼420 cm −1 appear to be asymmetric, which can be attributed to phonon confinement effects due to imperfect crystal quality [78,79]. The experimental spectra also reveal a peak at ∼154 cm −1 , as indicated by grey arrows, which is especially strong at 10 K and λ ex = 532 nm, and corresponds to the position of the A 1 mode in the LA branch at the M point. This mode is expected to be silent in first-order Raman processes since its |q| > 0. We can exclude that this peak is caused by higher-order Raman modes due to its low energy and, therefore, attribute its appearance to defect- activation that relaxes the q ≈ 0 selection rule [69,80].
The Raman spectra of Janus monolayer MoSSe are presented in Figure 2d-f, accompanied by the calculated Ph-DOS. Analogously to Janus monolayer WSSe, we compare the spectra to the theoretically predicted phonon energies (Figure 2f, dashed blue lines) and assign the firstorder Raman modes E 1 at ∼214 cm −1 for λ ex = 633 nm, Here, the spectra are strongly affected by the λ ex . First-order Raman modes, indicated by the blue arrows, are all visible only at λ ex = 633 nm, while barely two peaks are visible at λ ex = 532 nm. This arises once λ ex = 633 nm (1.96 eV) is close to the B exciton transition (bottom valence band to conduction band, see SI Figure S2), thereby increasing the Raman cross-section. Again, we generally observe good agreement between theory and experiment, well within the ∼8 cm −1 error of the calculations. Also for MoSSe, peaks appear around ∼150 cm −1 and ∼180 cm −1 for λ ex = 633 nm, as indicated by grey arrows, whose intensity is enhanced at 10 K. Similar to WSSe, the energy of the peaks in the ∼150 cm −1 range corresponds to the A 1 mode in the TA branch at the M point and to the A 1 and 2 E 1 modes in the TA, ZA branches at the K point. The peaks in the ∼180 cm −1 range correspond to the A 1 mode in the LA branch at the M point and the 1 E 1 mode at the K point in the LA branch. As discussed above, we exclude these peaks to be the result of higher-order Raman transitions due to their low energy, and we attribute their appearance to defectactivation. Interestingly, the presence of defect-activated Raman modes can be be used to monitor the defect concentration in the crystal, in analogy to the D peak in graphene [80], and as such constitutes precious information to assess crystal quality. The peak at ∼274 cm −1 corresponds to the 1 E 2 mode in the first ZO branch at K, however, due to the presence of other non-doubleresonant phonon combinations matching the same energy, its assignment requires further investigation. For completeness, all first-order Raman modes at Γ are summarized in Table 1.
To explore higher-order Raman peaks, we plot the Ra-   man spectra of Janus monolayers WSSe and MoSSe at 10 K between 100 and 800 cm −1 (Figure 3a and 3c) and compare them to twice the phonon energy of the Ph-DOS (Figure 3b and 3d). This is motivated by the role of double-resonant Raman scattering [81] in higher-order Raman transitions. In double-resonant Raman scattering, two phonons with the same momentum but opposite direction make electrons scatter far from their excitation point in the Brillouin Zone and then come back to the initial position, through two resonant and two non-resonant scattering events, satisfying q ≈ 0. Double-resonant Raman processes in WSSe and MoSSe are indicated by blue arrows in Figure 3a and 3c. Higher-order scattering processes that include phonons from different branches are also energetically allowed through defect activation that locally break crystal symmetries (dark grey arrows). A comprehensive assignment list of the observed higherorder Raman peaks is given in Table 2. The unassigned peaks which do not match with double-resonant processes may be defect-activated, however, further studies are required to elucidate their nature.
List of higher-order Raman modes of Janus monolayers WSSe and MoSSe, listed by experimental energy from spectra collected at λex = 633 nm ( † λex = 523 nm) and 10 K.

CONCLUSION
In summary, we presented a combined theoretical and experimental study of Raman modes of Janus monolayers WSSe and MoSSe. Excellent agreement was found for the frequencies of first-and higher-order Raman modes. Moreover, we discovered the presence of defect-activation of otherwise silent Raman modes, which may be used as markers for assessing crystal quality in further studies.
The recent synthesis of Janus monolayer TMDs adds an extra degree of freedom to the wider family of twodimensional and layered materials, with the presence of tuneable, strongly interacting dipolar excitons in singlelayer materials as the stepping-stone for further exploration of correlated many-body states, exciton transport and applications that exploit such features. However, novel physics and exciting new applications require indepth information over materials' properties and growth quality, with Raman spectroscopy being a widely utilized technique in such regard due to its descriptive power and simplicity of use and interpretation. Our work sets a general and much-needed reference over the vibrational properties of Janus monolayers, provides a starting point for further investigations on the role of phonons in such materials and enables the benchmarking of future crystal growth attempts.

DFPT
Phonon dispersion relations were calculated using density functional perturbation theory with the local-density approximation to the exchange-correlation function [82,83]. The vacuum distance between neighboring layers was 20Å to describe isolated layers within the periodic boundary conditions. Norm-conserving pseudopotentials and a basis set defined from a energy cutoff of 105 Ry [84,85] were used. The first Brillouin Zone was sampled with a 15×15×1 Monhorst Pack grid.

SEAR
The synthesis of Janus TMDs is carried out in a specially designed quartz chamber, and a home built inductively coupled plasma system. The plasma chamber consists of a 5-foot long quartz tube with a 1-inch inner diameter suspended off-centered on a Lindberg Blue/M singlezone furnace. A copper coil with a length ∼1.5 inches consisting of about five turns were wound around the quartz tube. The end of the Cu coil is connected to a 100 W tunable RF source (SEREN R101) through a customdesigned impedance match network. One end of the quartz tube is connected to an Edwards vacuum pump while the other end is fitted with a hydrogen supply line, hydrogen flow rate and the pressure within the chamber is regulated by means of a capacitance manometer and a pressure controller. For the synthesis of Janus TMDs, the reaction chamber is pumped down to a base pressure of 15 mTorr, after which the chamber was purged with 20 sccm H 2 flow, maintaining an operation pressure at 300 mTorr. Plasma was generated with 15 W RF power, and the visible plasma tail position was marked on the quartz tube. For the SEAR process to create WSSe, CVD grown WSe 2 was placed 4 cm upstream of marked visible plasma tail position onto a quartz boat, and 2 g sulfur was placed 15 cm upstream of the H 2 plasma tail. Plasma treatment lasted for 18 min. For the creation of MoSSe, the position of CVD-grown MoSe 2 and S source were kept at the same position as WSSe, except the processing time was decreased to 8 min because of the lower Mo-Se bond energy. The SEAR process can also be set up to create a 2D Janus structure from sulfur-based TMDs and selenium precursors in a similar fashion by varying the processing parameters.

Optical Setup
Raman and PL measurements were done using a custom-made confocal microscope in back-scattering geometry. The excitation laser beam is focused on the sample by an objective with N.A. = 0.75 to a diffractionlimited spot. For cryogenic measurements a He-flow cryostat (Cryovac) was used. The collected light is analyzed in a spectrometer (maximum point-to-point resolution ∼1.2 cm −1 with a grating 1200/mm lines) coupled to a Charged-Coupled Device (Horiba). Both incident (e i ) and scattered (e s ) light polarization vectors are placed in the xy-plane, which is relevant to account for the Raman intensity I = |e s Re i | 2 (for the case of no polarization filtering in the detection path, I = |Re i | 2 ) [63]. Here, R represents the Raman tensor, which is derived from the point group symmetry, and alongside the scattering configuration dictates the selection rules [86,87] (see SI  Table S4).

Supporting Information
Supporting information that accompanies this work: Symmetry and character tables; Phonon bandgaps; Photoluminescence of WSSe and MoSSe.

Notes
The authors declare no competing financial interest.
Supporting Information: The Raman Spectrum of Janus Transition Metal Dichalcogenide Monolayers WSSe and MoSSe S1. SYMMETRY AND CHARACTER TABLES Figure S1 shows the Brillouin Zone of the Janus TMDs. The zone centre Γ point has the same symmetry as the crystal, while the K and M high symmetry points are subgroups with C 3 and C s symmetries, respectively. Tables S1-S3 list the character table for each of these point groups. Table S4 shows the back-scattering geometry polarization selection rules.
FIGURE S1. Groups and subgroups for the high symmetry points of the Brillouin Zone of Janus TMDs.

S3. PHOTOLUMINESCENCE OF WSSe AND MoSSe
FIGURE S2. Photoluminescence spectra of Janus WSSe and MoSSe monolayers at 10 K and 300 K. Laser excitation wavelength λex = 633 nm is close to A exciton resonance in WSSe (dark blue) and to B exciton resonance in MoSSe (dark green).